1. Field of the Invention
This invention generally relates to a position control apparatus, and, in particular, to an X-Y-Z drive apparatus for controlling the position of a desired object in a three dimensional space.
2. Description of the Prior Art
An X-Y-Z drive apparatus for controlling the position of a desired object in a three dimensional space is well known in the art. A typical prior art X-Y-Z drive apparatus includes an X table assembly which is fixedly attached to a stationary object and which is elongated in shape in X direction. The X table assembly includes X guide rails extending in the X direction and a movable X table which is movable as guided by the X guide rails and on which a Y table assembly is fixedly mounted. Similarly, the Y table assembly includes Y guide rails extending in Y direction, which is perpendicular to the X direction, and a movable Y table which is movable as guided by the Y guide rails. In addition, a Z drive shaft assembly is fixedly attached to a side surface of the Y movable table and it includes a Z shaft which may move in Z direction which is perpendicular to both of X and Y directions. As a result, any desired object attached to the Z axis may be controlled in position in X, Y and Z directions.
However, in the above-described prior art X-Y-Z drive apparatus, since the Z drive shaft assembly is fixedly attached to a side surface of the movable Y table, the Y table always receive a moment due to the weight of the X drive shaft assembly. This is disadvantageous since undesired forces are applied to a guide mechanism interposed between the X and Y tables at all times so that a significant wear can occur prematurely. In addition, there is a limit in mounting an object on the Z drive shaft in controlling its position since the weight of such an object tends to increase the moment applied to the Y table.